Atomic Physics

346 Pages·2007·10.53 MB·English

OXFORD MASTER SERIES IN PHYSICS
OXFORD MASTER SERIES IN PHYSICS
The Oxford Master Series is designed for final year undergraduate and beginning graduate students in
physics and related disciplines. It has been driven by a perceived gap in the literature today. While basic
undergraduate physics texts often show little or no connection with the huge explosion of researchover the
last two decades, more advanced and specialized texts tend to be rather daunting for students. In this
series, all topics and their consequences are treated at a simple level, while pointers to recent developments
are provided at various stages. The emphasis in on clear physical principles like symmetry, quantum
mechanics, and electromagnetism which underlie the whole of physics. At the same time, the subjects are
related to real measurements and to the experimental techniques and devices currently used by physicists
in academe and industry. Books in this series are written as course books, and include ample tutorial
material, examples, illustrations, revision points, and problem sets. They can likewise be used as
preparation for students starting a doctorate in physics and related fields, or for recent graduates starting
research in one of these fields in industry.
CONDENSED MATTER PHYSICS
1. M. T. Dove: Structure and dynamics: an atomic view of materials
2. J. Singleton: Band theory and electronic properties of solids
3. A. M. Fox: Optical properties of solids
4. S. J. Blundell: Magnetism in condensed matter
5. J. F. Annett: Superconductivity
6. R. A. L. Jones: Soft condensed matter
ATOMIC, OPTICAL, AND LASER PHYSICS
7. C. J. Foot: Atomic physics
8. G. A. Brooker: Modern classical optics
9. S. M. Hooker, C. E. Webb: Laser physics
PARTICLE PHYSICS, ASTROPHYSICS, AND COSMOLOGY
10. D. H. Perkins: Particle astrophysics
11. Ta-Pei Cheng: Relativity, gravitation, and cosmology
STATISTICAL, COMPUTATIONAL, AND THEORETICAL PHYSICS
12. M. Maggiore: A modern introduction to quantum field theory
13. W. Krauth: Statistical mechanics: algorithms and computations
14. J. P. Sethna: Entropy, order parameters, and complexity
Atomic Physics
C. J. FOOT
Department of Physics
University of Oxford
1
3
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ISBN-10: 0 19 850695 3 (Hbk) Ean code 978 0 19 850695 9
ISBN-10: 0 19 850696 1 (Pbk) Ean code 978 0 19 850696 6
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Typeset by Julie M. Harris using LATEX
Printed in Great Britain
on acid-free paper by Antony Rowe, Chippenham
Preface
This book is primarilyintended to accompanyan undergraduatecourse
in atomic physics. It covers the core material and a selection of more
advanced topics that illustrate current research in this field. The first
six chapters describe the basic principles of atomic structure, starting
in Chapter 1 with a review of the classical ideas. Inevitably the dis-
cussion of the structure of hydrogen and helium in these early chapters
hasconsiderableoverlapwith introductoryquantummechanicscourses,
but an understanding of these simple systems provides the basis for the
treatment of more complex atoms in later chapters. Chapter 7 on the
interaction of radiation with atoms marks the transition between the
earlierchaptersonstructure andthe secondhalfofthe book whichcov-
ers laser spectroscopy, laser cooling, Bose–Einstein condensation of di-
lute atomic vapours, matter-waveinterferometryandiontrapping. The
exciting new developments in laser cooling and trapping of atoms and
Bose–EinsteincondensationledtoNobelprizesin1997and2001,respec-
tively. Some of the other selected topics show the incredible precision
thathasbeenachievedbymeasurementsinatomicphysicsexperiments.
Thistheme istakenupinthe finalchapterthatlooksatquantuminfor-
mation processing from an atomic physics perspective; the techniques
developed for precision measurements on atoms and ions give exquisite
control over these quantum systems and enable elegant new ideas from
quantum computation to be implemented.
Thebookassumesaknowledgeofquantummechanicsequivalenttoan
introductoryuniversitycourse,e.g.thesolutionoftheSchro¨dingerequa-
tioninthreedimensionsandperturbationtheory. Thisinitialknowledge
will be reinforced by many examples in this book; topics generally re-
garded as difficult at the undergraduate level are explained in some de-
tail, e.g. degenerate perturbation theory. The hierarchical structure of
atoms is welldescribed by perturbation theory since the different layers
ofstructurewithinatomshaveconsiderablydifferentenergiesassociated
withthem,andthisisreflectedinthenamesofthegross,fineandhyper-
fine structures. In the early chapters of this book, atomic physics may
appearto be simply appliedquantummechanics,i.e.we write downthe
Hamiltonian for a given interaction and solve the Schro¨dinger equation
with suitable approximations. I hope that the study of the more ad-
vanced material in the later chapters will lead to a more mature and
deeper understanding of atomic physics. Throughout this book the ex-
perimental basis of atomic physics is emphasised and it is hoped that
the reader will gain some factual knowledge of atomic spectra.
vi Preface
The selection of topics from the diversity of current atomic physics
is necessarily subjective. I have concentrated on low-energy and high-
precision experiments which, to some extent, reflects local research in-
terests that are used as examples in undergraduate lectures at Oxford.
One of the selection criteria was that the material is not readily avail-
able in other textbooks, at the time of writing, e.g. atomic collisions
have not been treated in detail (only a brief summary of the scattering
of ultracold atoms is included in Chapter 10). Other notable omissions
include: X-ray spectra, which are discussed only briefly in connection
with the historically important work of Moseley, althoughthey form an
important frontier of current research; atoms in strong laser fields and
plasmas; Rydberg atoms and atoms in doubly- and multiply-excited
states (e.g. excited by new synchrotron and free-electron laser sources);
and the structure and spectra of molecules.
IwouldliketothankGeoffreyBrookerforinvaluableadviceonphysics
(inparticularAppendixB)andontechnicaldetailsofwritingatextbook
for the Oxford Master Series. Keith Burnett, Jonathan Jones and An-
drew Steane have helped to clarify certain points, in my mind at least,
and hopefully also in the text. The series of lectures on laser cooling
givenbyWilliamPhillipswhilehewasavisitingprofessorinOxfordwas
extremely helpful in the writing of the chapter on that topic. The fol-
lowing people provided very useful comments on the draft manuscript:
Rachel Godun, David Lucas, Mark Lee, Matthew McDonnell, Martin
Shotter, Claes-Go¨ran Wahlstro¨m (Lund University) and the (anony-
mous) reviewers. Without the encouragementof So¨nke Adlung at OUP
this project would not have been completed. IrmgardSmith drew some
of the diagrams. I am very grateful for the diagrams and data supplied
by colleagues, and reproduced with their permission, as acknowledged
in the figure captions. Several of the exercises on atomic structure de-
rivefromOxfordUniversityexaminationpapersanditisnotpossibleto
identify the examiners individually—some of these exam questions may
themselves have been adapted from some older sources of which I am
not aware.
Finally, I would like to thank Professors Derek Stacey, Joshua Silver
andPatrickSandarswhotaughtmeatomicphysicsasanundergraduate
and graduate student in Oxford. I also owe a considerable debt to the
bookonelementaryatomicstructurebyGordonKembleWoodgate,who
was my predecessor as physics tutor at St Peter’s College, Oxford. In
writing this new text, I have tried to achieve the same high standards
ofclarityandconcisenessofexpressionwhilstintroducingnewexamples
and techniques from the laser era.
Background reading
It is not surprising that our languageshould be incapable of
describingtheprocessesoccurringwiththe atoms,foritwas
invented to describe the experiences of daily life, and these
consist only of processes involving exceeding large numbers
vii
of atoms. Furthermore, it is very difficult to modify our
languagesothatitwillbe abletodescribethese atomicpro-
cesses, for words can only describe things of which we can
form mental pictures, and this ability, too, in the result of
dailyexperience. Fortunately,mathematics isnotsubjectto
this limitation, and it has been possible to invent a mathe-
maticalscheme—the quantumtheory—whichseemsentirely
adequate for the treatment of atomic processes.
FromThe physical principles of the quantum theory, Werner
Heisenberg (1930).
The point of the excerpt is that quantum mechanics is essential for a
proper description of atomic physics and there are many quantum me-
chanicstextbooksthatwouldserveasusefulbackgroundreadingforthis
book. The followingshortlistincludes thosethatthe authorfoundpar-
ticularly relevant: Mandl (1992), Rae (1992) and Griffiths (1995). The
bookAtomicspectra bySoftley(1994)providesaconciseintroductionto
this field. The books Cohen-Tannoudji et al. (1977), Atkins (1983) and
BasdevantandDalibard(2000)areveryusefulforreferenceandcontain
many detailed examples of atomic physics. Angular-momentum theory
is very important for dealing with complicated atomic structures, but
it is beyond the intended level of this book. The classic book by Dirac
(1981) still provides a very readable account of the addition of angular
momentainquantummechanics. Amoreadvancedtreatmentofatomic
structure can be found in Condon and Odabasi (1980), Cowan (1981)
and Sobelman (1996).
Oxford C.J.F.
Web site:
http://www.physics.ox.ac.uk/users/foot
This site has answers to some of the exercises, corrections and other
supplementary information.
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Contents
1 Early atomic physics 1
1.1 Introduction 1
1.2 Spectrum of atomic hydrogen 1
1.3 Bohr’s theory 3
1.4 Relativistic effects 5
1.5 Moseley and the atomic number 7
1.6 Radiative decay 11
1.7 Einstein A and B coefficients 11
1.8 The Zeeman effect 13
1.8.1 Experimental observation of the Zeeman effect 17
1.9 Summary of atomic units 18
Exercises 19
2 The hydrogen atom 22
2.1 The Schro¨dinger equation 22
2.1.1 Solution of the angular equation 23
2.1.2 Solution of the radial equation 26
2.2 Transitions 29
2.2.1 Selection rules 30
2.2.2 Integration with respect to θ 32
2.2.3 Parity 32
2.3 Fine structure 34
2.3.1 Spin of the electron 35
2.3.2 The spin–orbit interaction 36
2.3.3 The fine structure of hydrogen 38
2.3.4 The Lamb shift 40
2.3.5 Transitions between fine-structure levels 41
Further reading 42
Exercises 42
3 Helium 45
3.1 The ground state of helium 45
3.2 Excited states of helium 46
3.2.1 Spin eigenstates 51
3.2.2 Transitions in helium 52
3.3 Evaluation of the integrals in helium 53
3.3.1 Ground state 53
3.3.2 Excited states: the direct integral 54
3.3.3 Excited states: the exchange integral 55